1. Field of the Invention
This invention relates to a tank drying and ventilation system and, more particularly, to a system for drying and ventilating jet fuel tanks for reducing the fuel vapor level from residual fuel in an emptied fuel tank so that the fuel tank may be safely entered by maintenance personnel.
2. Description of the Prior Art
The components of a jet aircraft, including the fuel tanks, undergo considerable stress and flexure during flight. As a result, the fuel tanks often develop leaks. It then becomes necessary for maintenance personnel to enter a tank to repair the leaks. The tank must first be purged of all fuel (typically, Jet A jet fuel). However, after the fuel has been drained from the tank, residual fuel remains, producing a level of fuel fumes which makes the atmosphere within the tank unsafe. Various methods have been suggested for accelerating the drying time of large aircraft fuel tanks after the drainage of Jet A jet fuel. For example, Boeing Aircraft Company recommends fuel tank ventilating equipment that consists of a tank access port cover with a flexible hose adaptor connected to a commercially available blower. The exhaust of the blower must, however, be ventilated to a safe area due to the explosion hazard of the vapor of Jet A jet fuel. The vapor content of the exhaust from the tank is monitored, and the drying process is discontinued when the hazardous vapor level is less than 300 parts per million. This equipment takes too long, typical cycle time being twenty-four hours, and during the drying cycle, the hazardous exhaust situation must be considered as well as EPA regulations.
There are also some permanent hangar facilities in present use with large roof mounted ventilators, and the required adaptors to the fuel tanks. These installations are costly, limit fuel tank servicing to the hangar environment, and do not address the potential problems with EPA regulations. The systems generally exhaust the pollutants to the atmosphere, probably in violation of most such regulations.
In another context, U.S. Pat. No. 3,973,201 discloses a vapor recovery system for use with a fuel tank as it is filled with fuel. A refrigeration unit condenses fuel vapors and the portion of vapor which is not condensed is recirculated back through the refrigeration unit. The condensed fuel is returned to the tank. Periodically, the system is vented to the atmosphere through a canister filled with activated carbon. It will be recognized, however, that the system of this patent does not address the problem of reducing the fuel vapor level in an emptied fuel tank.